Mechanics, Physics, Chemistry and Shale Rock Properties

The fundamental mechanics, physics and chemistry of shale behaviour remain poorly understood, partly due to neglect and partly due to poor experimental practice. This paper will review the role of water content, water saturation and cation exchange capacity (CEC) on shale mechanical and petrophysical properties. Decreasing water content increases shale strength and static Young’s modulus under both fully and partially saturated conditions. Partial saturation increases S-wave velocity but decreases P-wave velocity, so the dynamic stiffness response depends on the particular modulus in question – shear and Young’s increase while bulk decreases. CEC correlates well with rock strength (cohesion, friction) and is also strongly correlated to parameters such as the reflection coefficient (membrane efficiency) and the osmotic chemomechanical coupling coefficient. CEC is also directly related to dielectric dispersion and holds the tantalising possibility of estimating shale strength from dielectric logs although more work is required across the different scales of investigation. Finally, the dielectric response of shales seems to correlate very well with P-wave velocity at certain frequencies. Hence, the mechanical, physical and chemical properties of shales are all intimately interlinked, requiring multi-disciplinary approaches to unravel the fundamentals behind the processes occurring in these rocks.